1. Field of the Invention
The present invention relates to the art and science of papermaking. More particularly, the invention relates to a method and apparatus for measuring paper web properties while in situ of the paper machine.
2. Description of the Prior Art
Throughout the production of a fourdrinier machine paper web, certain quality and property characteristics of the web are continuously monitored and measured. Among such monitored properties are those relating to surface smoothness, optical uniformity and formation quality.
Paper web formation quality is that characteristic of paper that is manually ascertained by the uniformity of light transmittance through a back lighted web. A mottled sheet is of poor formation quality. The characteristic is objectively measured by detecting variations in light transmittance through a given length of web from a constant intensity source. Maintenance of a constant intensity light source requires a constant physical displacement position of the web from the light source within a fixed separation gap between the light source and light detector. At a web production velocity of 1500 feet per minute and greater, the web begins to flutter and float at unsupported locations along the machine length. Necessarily, detector heads of measuring instruments must be located at unsupported web locations.
As a general rule, no stationary structure is permitted to have contact with the traveling paper machine web. This practicer has numerous justifications including a potential for marking or scoring the web, as a machine threading obstruction and as a surface of debris accumulation and web contamination.
Resultantly, the accuracy and reliability of web property measuring instruments such as formation testers and monitors is greatly compromised on high speed paper machines due to a heretofore conflict between the measuring requirement to hold a constant web position proximity and the machine operating discipline that denies the presence of structure necessary to confine the web position.
It is, therefore, an object of the present invention to provide a method and apparatus for maintaining the proximity of an in situ paper machine web to a web property measuring unit.
Another object of the present invention is to provide a method and apparatus that secures a point position of an in situ paper machine web without structural contact.
Another object of the present invention is to improve the accuracy of formation monitors and other in situ web property measuring instruments by stabilizing the web location relative to the instrument measuring means.